A group of research problems all involving the antibiotic family named the erythromycins is in progress. Those supported by Grant AI 09 158-06 with three major areas of the biochemistry or pharmacology of this group of drugs. One approach is to define further the metabolic relationships that exist among the four known erythromycins. Currently, a study of erythromycin D, a newly discovered member of the group is under way. This erythromycin (D) is a logical precursor of all the other known erythromycins and work on the enzymes that carry out these conversions is in progress. Two transmethylation steps and one hydroxylation reaction are involved, and characterization of the transmethylase is nearly complete. The hydroxylase is also under study. Related to the hydroxylase that presumably converts erythromycin D into C may be another (the same?) hydroxylase that transforms the penultimate sugar-free lactone known to the form which is the acceptor of the first of the two sugars present in the erythromycins. This hydroxylase is a cytochrome P 450-linked mixed-function oxydase and is present in the soluble fraction of Streptomyces erythreus, the producer of the erythromycins. No specific induction of the hydroxylase is needed, and these facts taken together with the unique nature of the substrate (6- deoxyerythronolide B) make further study of this hydroxylase of great interest. The third problem currently being studied is that of the human metabolism of erythromycin A, the therapeutic form of erythromycin. Urinary metabolites are being fractionated and the compounds present are being characterized. A number of new and unpredicted substances appear to be present, and the present aim of the study is to determine the nature and constancy of the pattern, following which an attempt will be made to find an animal model system which yields similar metabolic products. If successful, the model will be used in the evaluation of chemically modified forms of erythromycin which are not suitable for study in man.